The importance and difficulty associated with fan blade containment has increased with the introduction of larger and more powerful turbofan engines into the aircraft industry. Preventing the escape of a fan blade subsequent to the blade becoming detached from the rotor assembly presents structural, size and weight challenges.
A typical containment assembly includes a rigid cylindrical shell surrounded by a plurality of cloth wraps. The shell or casing is formed from a stiff structural material. The cloth wraps are usually formed from a high strength material, such as KEVLAR (KEVLAR is a registered trademark of Dupont Corporation). A detached fan blade is thrown outward and passes through the case but is caught by the cloth wraps. The cloth wraps deflect in response to the impact but retain the fan blade. The case then provides a bearing surface to support the unbalanced array of fan blades. For shipping and handling considerations, it is desired that the radial profile of the case be as small as possible.
Spaced outward of the case and cloth wraps is the nacelle. The wraps are designed to deflect an amount sufficient to contain the blade but not to permit impact of the blade with the nacelle to prevent damage to the nacelle. The amount of radial spacing required between the wraps and the nacelle contributes to the radial profile of the nacelle. For aerodynamic considerations, it is desirable to have the radial profile of the nacelle be as small as possible. The spacing may be made smaller by increasing the number of wraps but at the cost of adding weight to the containment assembly. The additional weight negatively impacts the operating efficiency of the powerplant. As a result, there is a trade-off between the radial profile of the nacelle and the weight of the containment assembly.
In addition to the case and cloth wraps, the containment assembly also includes a liner defined by a plurality of circumferentially adjacent acoustic panels. The liner extends the length of the case and includes a flow surface that faces inwards towards the array of fan blades. Since the flow surface is non-linear, the liner provides a structure that is easily shaped, as opposed to the case, to form the flow surface.
The acoustic panels are typically a honeycomb type of material that minimize noise levels associated with the fan. Engine noise is another area that has increased in importance in recent years. In this respect, the Environmental Protection Agency issues regulations on the permissible noise levels of aircraft engines.
Unfortunately, providing the necessary amount of acoustic treatment over the inner surface of the case has several drawbacks. First, for a given fan blade length it increases the diameter of the containment assembly and thereby the radial profile of the case and nacelle. Second, it increases the separation between the fan blades and the inner surface of the case which, as described above, acts as a bearing surface in the event of a blade loss. The greater the separation between the fan blades and the bearing surface, the greater the radial movement of the rotor assembly and the higher the risk of further damage to the rotor assembly in the event the rotor assembly becomes unbalanced.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop effective containment assemblies that are lightweight and have small radial profiles for aircraft engines.